Intake and exhaust cycles overlap in two-cycle engines. Through a range of piston positions, fresh air and fuel enter the combustion chamber through an intake port at the same time that exhaust gases leave the combustion chamber through a regulator port. Exhaust gas back pressures from an exhaust system can affect both cycles.
Different objectives can be set for two-cycle engine performance and different exhaust back pressures can be optimal for these different objectives. Optimal back pressures also tend to vary over the operating range of two-cycle engines. Most exhaust systems adopt a geometric form for optimizing both dynamic and static exhaust back pressures. For example, specially shaped exhaust conduits have been developed to manage pressure pulses associated with the cyclical nature of the exhaust discharges. Constricted exhaust flow paths, such as provided by limited size outlet pipes, can be used to affect overall exhaust back pressures.
Valves for controlling resistance to exhaust flows have also been used. Some such valves have been arranged to maintain more constant back pressures over a range of engine speeds. Others have sought to lower resistance to exhaust flows at low speeds or light load operations. For example, U.S. Pat. No. 4,051,821 to Amann mounts a butterfly valve along an exhaust passage. A hydraulic control system powered by an engine-driven oil pump moves the butterfly valve between open and closed positions as a result of a feedback pressure in the exhaust passage.
U.S. Pat. No. 3,969,895 to Krizman discloses an exhaust flow regulating valve operated by back pressures within an exhaust conduit. The valve includes a closure plate that is biased against an open end of the exhaust conduit. A perforated tube carried by the closure fits inside the open end of the exhaust conduit. Exhaust back pressures displace the closure plate allowing exhaust gases to discharge through and around the perforated tube to atmosphere. A spring bias controls the amount the closure plate and perforated tube are displaced by the exhaust back pressures.
My earlier U.S. Pat. No. 5,785,014 provides for further regulating exhaust back pressures of two-cycle engines by combining a biased back pressure sensitive piston with a cross-sectional flow area that varies non-linearly with displacements of the piston. In particular, the cross-sectional flow area of the exhaust passageway increases non-linearly in response to back pressure induced displacements of the piston. Overall, the exhaust system significantly reduces restriction to airflow as back-pressure otherwise increases.